I was stumbling around Google's Trend page the other day trying to get an idea of just how much headway other distros like Suse were making on Ubuntu. Turns out that most searches are still for Ubuntu.. actually, according to Google, that trend seems to be growing. Other search trends seemed awfully static... for example Suse and FreeBSD seemed steady for the most part. The search term "linux" however looks to be declining. I attribute that mostly to the fragmentation of the linux community and how searching for distro-specific help has become more important.
Case in point, you don't need to know how to install rails on linux, you need to know how to install rails on Ubuntu Intrepid Ibex. Package managers make it much easier than building packages manually from source and since many distros have different package managers the process is often different.
Anyway, check out this image of what the trend looks like as of November 25, 2008. Pretty interesting to see that as linux drops ubuntu gains. Not sure what it infers exactly.. not enough data there to really conclude anything, but if you hop on Google Trends yourself and compare linux with any other distro name you won't see convergence like this!